1. Field of the Invention
This invention relates to film scanners, and more particularly, to frame-rate converting scanners for scanning a motion picture film projected at a selected frame rate, e.g. 18 frames per second, to produce a television signal at a standard field rate, e.g. 60 fields per second.
2. Discussion Related to the Problem
Apparatus for scanning motion picture film to produce a television signal is well known. In such film scanning apparatus, a film frame is scanned in a direction generally perpendicular to the length of the film at a standard television line rate. The line scan is displaced in a direction generally parallel with the length of the film at a standard television field rate. In the past, this scanning has been accomplished by a flying spot scanner such as a cathode ray tube. The use of a solid state image sensing array as an alternative to a CRT has been proposed. See, D. T. Wright, "Solid State Sensors; The Use of a Single Dimension 512-Element Array for Film Scanning", BBC Research Department Report No. 1973/32. The potential advantages of a solid state sensing array over a flying spot scanner are numerous, including: reduced size and weight; greater reliability; and lower cost. These advantages made a solid state image sensing array potentially attractive for use in a film scanner for displaying amateur movies on a home television set. However, since standard motion picture film frame rates are not the same as standard television field rates, the scanning apparatus must provide some form of frame-rate conversion, whereby the film is projected at a desired projection rate while the frames are scanned at a standard television field rate. It is also desirable in such a film scanner to provide several alternative projection rates such as stop action, slow motion, and fast forward.
One approach to frame-rate conversion suggested in the prior art is to scan the film one frame at a time at the projection rate, while storing the scanned information in a digital field store memory. The signal thus stored is repeatedly read out of the field store at the standard television rate a sufficient number of times to accomplish frame-rate conversion. Unfortunately, the addition of a digital field store memory and the attendant analog-to-digital converters and clocking means required to coordinate the read out of the sensor for projection rates that are not integer submultiples of the field rate greatly increases the cost and complexity of the scanning apparatus. Alternatively, it has been suggested to interpose a moving optical device such as a pivoting mirror or rotating optical polygon in the optical path between the film and the solid state sensing array, and to move the device in such a manner as to cause the array to scan the projected image of the film a plurality of times. See, I. Childs and J. Sanders, "An Experimental Telecine Using a Line-Array CCD Sensor," SMPTE Journal, Vol. 87, No. 4, April 1978. The addition of moving optics to the scanning system increases the cost of the system and introduces such problems as alignment, vibration, and extreme manufacturing tolerances on the optical surfaces of the moving element. Furthermore, this scheme may work very nicely when the field rate is a whole number multiple of the projection rate, but complications arise when it is otherwise. The goal of the inventors, therefore, was to provide a frame-rate converting film scanner capable of accomplishing several projection rates while avoiding the use of costly digital field store memories or moving optical elements.